1. Field of the Invention
The present invention relates to an edge flaw inspection device that optically inspects flaws in inspected edges, and more particularly, to a device that detects flaws in components formed into the shape of plates such as silicon wafers and semiconductor wafers.
2. Background Art
Edge flaw inspection devices that detect edge flaws such as cracks, chips or polishing marks in long, narrow edges such as the outer edges of silicon wafers have a structure described in, for example, Japanese Patent No. 2999712 (Japanese Patent Application, First Publication No. Hei 9-269298).
As shown in FIG. 6, this edge flaw inspection device 20 is provided with an elliptical mirror 23 in the form of a concave mirror that is severed near the midpoint of a first focal point 21 and a second focal point 22, a light source 24 that radiates laser light towards first focal point 21 of elliptical mirror 23, a photo detector 25 arranged at second focal point 22, and a douser 26 arranged between first focal point 21 and light source 24. A slit 27 is formed along a horizontal plane that contains two focal points 21 and 22 (the direction of this plane is hereinafter to be the horizontal direction) in the apex of elliptical mirror 23. Reference symbol 30 in the drawing indicates a lens.
Slit 27 has a width that is slightly larger than the thickness of a silicon wafer 28, and a portion of silicon wafer 28 can be inserted within elliptical mirror 23 from outside elliptical mirror 23. Silicon wafer 28 that has been inserted into elliptical mirror 23 from slit 27 is held so that its outer peripheral edge does not pass first focal point 21. In addition, silicon wafer 28 is rotatably held about a vertical axis, and the outer peripheral edge arranged at first focal point 21 can be continuously changed in the peripheral direction.
Through hole 29 is formed in douser 26 on the light path that connects light source 24 and first focal point 21. Laser light emitted from light source 24 passes through through hole 29 of douser 26, and reaches the outer peripheral edge of silicon wafer 28 arranged at first focal point 21 where it is then reflected.
In the case of viewing silicon wafer 28 with its edge facing towards the front, there is a flaw in its edge as shown in FIG. 7, and if this flaw is a vertical flaw 31 that extends in the vertical direction, laser light radiated onto the edge is typically strongly scattered to the left and right. On the other hand, if a flaw that has occurred in the edge is a horizontal flaw 32, laser light radiated onto the edge is typically strongly scattered up and down.
This reflected light scattered at the peripheral edge of silicon wafer 28 at first focal point 21 is scattered three-dimensionally, reaches the mirrored surface of elliptical mirror 23, and is then reflected there after which it converges at second focal point 22. Since photo detector 25 is arranged at second focal point 22, the converted scattered and reflected light is detected by photo detector 25.
Since this scattered reflected light has different frequency components depending on the type of defect present in the peripheral edge of silicon wafer, its surface roughness and so forth, by detecting this light and analyzing its frequency components, the type of defect, surface roughness and so forth can be detected. In addition, as a result of rotating silicon wafer 28 about a vertical axis, its peripheral edge can be detected for flaws over its entire circumference.
On the other hand, light reflected from the surface other than the location of a flaw in the outer peripheral edge of silicon wafer 28 is in the form of low order diffracted light such as so-called regular reflected light. For example, regular reflected light from the outer peripheral edge of silicon wafer 28 arranged along the horizontal direction is predominantly light that is reflected in the vertical plane that contains first focal point 21 and second focal point 22 or its vicinity. However, since this low order diffracted light is light reflected by a surface other than that containing a flaw, and does not contain information necessary for detecting a flaw in the outer peripheral surface, it is preferable that it not be detected by photo detector 25.
In an edge flaw detection device 20 of the prior art, a douser 26 is provided in the form of a strip that extends in the vertical direction in the space between light source 24 and first focal point 21, which either prevents regular reflected light and other low order diffracted light from reaching the mirrored surface of elliptical mirror 23, or blocks low order diffracted light that has reached the mirrored surface of elliptical mirror 23 from reaching the second focal point after it has been reflected there.
However, in the case of a douser 26 arranged in the space between a light source and a first focal point as was previously described, the problem arises in which not only low order diffracted light such as regular reflected light, but also all diffracted light that attempts to pass through the space in which douser 26 is arranged ends up being blocked.
Namely, all diffracted light reflected by the outer peripheral edge of silicon wafer 28 is three-dimensionally scattered at first focal point 21, and proceeds through the plane that contains the scattering direction vector and the first and second focal points 21 and 22. However, in a device of the prior art in which douser 26 is arranged in the space between light source 24 and first focal point 21, even in the case of scattered diffracted light scattered by an edge flaw in the outer peripheral edge of silicon wafer 28, if the scattering direction vector is facing in a direction at an angle that is shallower than the vertical plane as in the manner of scattered diffracted light produced by a horizontal flaw, the plane through which the scattered diffracted light proceeds ends up intersecting with douser 26, thereby resulting in the problem of the scattered diffracted light being blocked by douser 26.
In this case, since diffracted light containing effective information relating to edge flaws cannot be detected by photo detector 25, there were cases in which it was difficult to judge the presence and types of defects.
In addition, as indicated in Japanese Unexamined Patent Application, First Publication No. 11-351850, although a method has been proposed in which regular reflected light is blocked and scattered diffracted light produced by a horizontal flaw is detected by arranging a light receiving element array in the vicinity of a first focal point, since the light receiving element array arranged in the space inside an elliptical mirror blocks scattered diffracted light effective for flaw detection, there was the problem of a decrease in the amount of information detected by a photo detector in the same manner as described above.
In consideration of the above circumstances, an object of the present invention is to provide an edge flaw inspection device that effectively eliminates low order diffracted light such as regular reflected light, but detects high order diffracted light such as scattered diffracted light in a photo detector without blocking that light.
An edge flaw inspection device of the present invention comprises: an elliptical mirror having a first focal point and a second focal points; a light source that radiates coherent light towards an inspected edge arranged near the first focal point of the elliptical mirror; a light blocking member that blocks diffracted light of a low order that is radiated from the light source and reflected by the inspected edge; and a photo detector arranged at the second focal point of the elliptical mirror; and the light blocking member comprising a light absorbing member arranged on the mirrored surface of the elliptical mirror reached by the low order diffracted right.
According to the present invention, coherent light emitted from a light source is reflected by an inspected edge. Since the inspected edge is arranged near a first focal point, the light is scattered three-dimensionally according to the status of the inspected edge. Since an elliptical mirror is arranged in the direction of scattering, light reflected by the mirrored surface of that elliptical mirror is diffracted towards the direction of a second focal point. Since a photo detector is provided at the second focal point, all of the reflected light converges at the second focal point where it is then detected.
In this case, although diffracted light of a low order that does not contain flaw information is present from the inspected edge, since the direction in which this low order diffracted light scatters is fixed to a certain extent, and a light blocking member composed of a light absorbing material is provided in the mirrored surface of the elliptical mirror arranged in the direction of scattering, low order diffracted light is absorbed by the light absorbing material, and is prevented from being detected by the photo detector. On the other hand, although other diffracted light is reflected by the mirrored surface of the elliptical mirror, since the light blocking member is only arranged on the mirrored surface, and is not arranged in the space inside the elliptical mirror as in the prior art, all scattered refracted light reflected by the mirrored surface of the elliptical mirror converges at the second focal point and is detected by the photo detector.
The light absorbing member may have a width corresponding to the distance from the first focal point. According to this aspect, although light reflected at the first focal point is reflected three-dimensionally in all directions, regular reflected light and other low order diffracted light that does not contain flaw information is also scattered over a prescribed angle range. Namely, light reflected over a prescribed angle range at the first focal point has a narrow width in the vicinity of the first focal point, that width increases the greater the distance from the first focal point. Thus, by making the width of the light absorbing material narrow close to the first focal point, and then increasing the width as the distance from the first focal point increases, low order diffracted light not required for detecting edge flaws is effectively absorbed, while high order diffracted light required for flaw detection can be effectively oriented towards the second focal point. The width of each part of the light absorbing member may be adjusted proportional to the distance from the first focal point to the each part.
The light absorbing member may be a masking tape composed from a light absorbing material. According to this invention, an edge flaw inspection device can be composed extremely easily by using masking tape for the light absorbing material. In addition, as a result of using masking tape, the light absorbing material can be easily attached to and removed from the elliptical mirror, and by adjusting the width of the masking tape, a device can be provided that can be matched to the type of edge flaw desired to be detected.
The inspected edge may be the periphery of a disk-shaped inspection target, and a focusing member is provided that forms an irradiated spot of coherent light formed at the inspected edge in a shape that is shorter in the peripheral direction than the thickness direction of the inspection target. According to this aspect, by inspecting a disk-shaped inspection target over a wide range in the direction of thickness, and narrowly focusing laser light in the peripheral direction, fine edge flaws can be effectively detected.
The focusing member may be one of a cylindrical lens, a rod lens, or a toric lens having a different radius of curvature in two directions.